Super-diffusion of Photoexcited Carriers in Topological Insulator Nanoribbons

TL;DR

This study reveals ultrafast, long-range super-diffusion of photoexcited carriers in topological insulator nanoribbons, with diffusivity up to 800 cm$^2$/s at low temperatures, highlighting potential for optical spin manipulation.

Contribution

It demonstrates the significant increase in carrier diffusivity and long-range transport in topological insulator nanoribbons, providing insights into super-diffusion mechanisms and exciton effects.

Findings

Carrier diffusivity reaches 800 cm$^2$/s at 21 K.

Photoexcited carriers travel up to 10 μm for hundreds of picoseconds.

Diffusivity peaks in intrinsic devices and decreases at high temperatures.

Abstract

Understanding the ultrafast dynamics and transport of photoexcited carriers in topological insulators is crucial for the optical manipulation of spins and may shed light on the nature of topological excitons. Here we investigate bulk-insulating Sb-doped $\mathrm{Bi_2Se_3}$ nanoribbons via ultrafast transient photovoltage microscopy. The probe-pulse-induced photovoltage is substantially suppressed by a pump pulse. Recovery time increases from 50 to 1600 picoseconds as the pump fluence increases. We found that the diffusivity of photoexcited carriers increases significantly at lower carrier concentrations, up to 800 cm$^2$/s at 21 K, two to three orders of magnitude higher than that of band-edge carriers. Remarkably, the photoexcited carriers travel up to 10 $\mu$m for hundreds of picoseconds at this high diffusivity. The diffusivity peaks in intrinsic devices and is reduced at high temperatures. We discuss the possible mechanisms of long-ranged super-diffusion in the frames of hot carriers and exciton condensation.